When setting up for defensive battles, I often use the "F" (Fire) key to check line-of-sight. When I've seen what I want to see, I hit "Esc" to cancel the "F".

A couple of times while doing this, I've accidentally given an actual fire order by inadvertently left-clicking with the mouse somewhere on the map. One this happens, the soldiers from the unit change to "firing" status and there's no way to cancel this.

What happens then is that, as soon as I start the game, that unit opens fire at the point I accidentally clicked on. I can quickly hit "Cease Fire" but usually not fast enough to stop the unit from firing at least one or two shots before I do so.

The neat thing I've noticed is that the enemy reacts to this fire, even if it can't see where it's coming from. It will sometimes target the location of the firing unit with artillery and will move units towards that position. It could be a useful way of suckering the AI during defensive games.